8.12.1 Optimizing Disk I/O

This section describes ways to configure storage devices when
you can devote more and faster storage hardware to the
database server. For information about optimizing an
InnoDB configuration to improve I/O
performance, see Section 8.5.8, “Optimizing InnoDB Disk I/O”.

Disk seeks are a huge performance bottleneck. This problem
becomes more apparent when the amount of data starts to grow
so large that effective caching becomes impossible. For
large databases where you access data more or less randomly,
you can be sure that you need at least one disk seek to read
and a couple of disk seeks to write things. To minimize this
problem, use disks with low seek times.

Increase the number of available disk spindles (and thereby
reduce the seek overhead) by either symlinking files to
different disks or striping the disks:

Using symbolic links

This means that, for MyISAM tables,
you symlink the index file and data files from their
usual location in the data directory to another disk
(that may also be striped). This makes both the seek and
read times better, assuming that the disk is not used
for other purposes as well. See
Section 8.12.2, “Using Symbolic Links”.

Symbolic links are not supported for use with
InnoDB tables. However, it is
possible to place InnoDB data and log
files on different physical disks. For more information,
see Section 8.5.8, “Optimizing InnoDB Disk I/O”.

Striping

Striping means that you have many disks and put the
first block on the first disk, the second block on the
second disk, and the N-th
block on the (N MOD
number_of_disks)
disk, and so on. This means if your normal data size is
less than the stripe size (or perfectly aligned), you
get much better performance. Striping is very dependent
on the operating system and the stripe size, so
benchmark your application with different stripe sizes.
See Section 8.13.2, “Using Your Own Benchmarks”.

The speed difference for striping is
very dependent on the parameters.
Depending on how you set the striping parameters and
number of disks, you may get differences measured in
orders of magnitude. You have to choose to optimize for
random or sequential access.

For reliability, you may want to use RAID 0+1 (striping plus
mirroring), but in this case, you need 2 ×
N drives to hold
N drives of data. This is
probably the best option if you have the money for it.
However, you may also have to invest in some
volume-management software to handle it efficiently.

A good option is to vary the RAID level according to how
critical a type of data is. For example, store
semi-important data that can be regenerated on a RAID 0
disk, but store really important data such as host
information and logs on a RAID 0+1 or RAID
N disk. RAID
N can be a problem if you have
many writes, due to the time required to update the parity
bits.

You can also set the parameters for the file system that the
database uses:

If you do not need to know when files were last accessed
(which is not really useful on a database server), you can
mount your file systems with the -o noatime
option. That skips updates to the last access time in inodes
on the file system, which avoids some disk seeks.

On many operating systems, you can set a file system to be
updated asynchronously by mounting it with the -o
async option. If your computer is reasonably
stable, this should give you better performance without
sacrificing too much reliability. (This flag is on by
default on Linux.)

Using NFS with MySQL

You should be cautious when considering whether to use NFS
with MySQL. Potential issues, which vary by operating system
and NFS version, include the following:

MySQL data and log files placed on NFS volumes becoming
locked and unavailable for use. Locking issues may occur
in cases where multiple instances of MySQL access the same
data directory or where MySQL is shut down improperly, due
to a power outage, for example. NFS version 4 addresses
underlying locking issues with the introduction of
advisory and lease-based locking. However, sharing a data
directory among MySQL instances is not recommended.

Data inconsistencies introduced due to messages received
out of order or lost network traffic. To avoid this issue,
use TCP with hard and
intr mount options.

Maximum file size limitations. NFS Version 2 clients can
only access the lowest 2GB of a file (signed 32 bit
offset). NFS Version 3 clients support larger files (up to
64 bit offsets). The maximum supported file size also
depends on the local file system of the NFS server.

Using NFS within a professional SAN environment or other
storage system tends to offer greater reliability than using
NFS outside of such an environment. However, NFS within a SAN
environment may be slower than directly attached or
bus-attached non-rotational storage.

If you choose to use NFS, NFS Version 4 or later is
recommended, as is testing your NFS setup thoroughly before
deploying into a production environment.